Gold Nanoparticle Mesoporous Carbon Composite as Catalyst for Hydrogen Evolution Reaction

Increased environmental pollution and the shortage of the current fossil fuel energy supply has increased the demand for eco-friendly energy sources. Hydrogen energy has become a potential solution due to its availability and green combustion byproduct. Hydrogen feedstock materials like sodium borohydride (NaBH4) are promising sources of hydrogen; however, the rate at which the hydrogen is released during its reaction with water is slow and requires a stable catalyst. In this study, gold nanoparticles were deposited onto mesoporous carbon to form a nano-composite catalyst (AuNP-MCM), which was then characterized via transmission electron microscopy (TEM), powder X-ray diffraction (P-XRD), and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The composite’s catalytic ability in a hydrogen evolution reaction was tested under varying conditions, including NaBH4 concentration, pH, and temperature, and it showed an activation of energy of 30.0 kJ mol−1. It was determined that the optimal reaction conditions include high NaBH4 concentrations, lower pH, and higher temperatures. This catalyst, with its stability and competitively low activation energy, makes it a promising material for hydrogen generation.


Introduction
Hydrogen fuel is an alternative type of fuel that has potential in solving the world's energy crisis [1].The energy generation of gasoline combustion is lower than the energy released from hydrogen combustion [2].When hydrogen is used as a fuel, the major byproduct, in terms of its energy reaction, is water, so its environmental impact is minimal [2].Furthermore, hydrogen can be generated from the reaction between hydrogen feedstock, such as sodium borohydrides (NaBH 4 ), and water [3], as seen in Equation (1).The widespread implementation of hydrogen as a fuel would reduce dependence on fossil fuels; however, the biggest disadvantage of this reaction is the slow rate of reaction between metal hydrides and water.As such, a catalyst is necessary before hydrogen gas can become a viable fuel source [3].
NaBH 4 + 2H 2 O → 4H 2 + NaBO 2 (1) Among the catalysts, nanoparticles have been the focus of much scientific study due to their unique properties, including their catalytic properties [4,5]; however, it is difficult to control nanoparticle performance in reactions as this depends greatly on characteristics such as shape, size, crystal structure, and texture [6][7][8].The agglomeration of nanoparticles often affects their size and structure and leads to the degeneration of catalytic ability [9,10].In order to mitigate this issue, the nanoparticles can be imbedded on a carbon template to prevent their agglomeration and improve the durability of the material in catalytic reactions [5,11].
One family of durable carbon materials that also has catalytic potential is that of mesoporous carbon materials (MCMs) [12,13].The different types of porous carbons are characterized by their differences in pore size, with mesoporous carbon materials having a pore size 2-50 nm, while microporous and macroporous carbon have smaller and larger pore sizes, respectively [14,15].The narrow pore size range allows for significant control of various characteristics during synthesis, which includes thermal, mechanical, and electrical stability, chemical inertness, ordered pore structure, large surface area, pore volume, and catalytic activity [16,17].Most traditional metal and biological catalysts have several disadvantages, such as high reaction temperature, long reaction time, low conversion ratio, and low regioselectivity, so research is being conducted with MCMs to reduce these disadvantages [18].For example, factors like cost, complexity of synthesis, and potential wasted materials can be reduced by using MCMs as opposed to traditional catalysts as MCMs are only made of ordered carbon atoms that are synthesized via well-established procedures [19].
One such method of synthesizing MCMs involves filling mesoporous silica with a carbon precursor like sucrose, which undergoes several high-temperature processes.The silica is removed with hydrofluoric acid, which leaves only carbon in the form of MCMs [20].Unfortunately, this process involves aggressive chemicals that, along with intermediary products, become dangerous waste products.One novel method of forming MCMs has recently been developed, which involves using starches and expanding the present pores to increase the catalytic potential [21].Starches are typically high-density substances, which makes them catalytically inert; but by expanding the pores, this activity was increased [21].The pore expansion temperature is the key to controlling pore size and volume, with a similar process used on plant fibers resulting in similar results [21,22].The benefits of this new method include the starting material being completely nontoxic and renewable, the lack of toxic materials needed for the reaction, and the ease of synthesis [21,23,24].

Catalyst Characterization
The gold nanoparticles for the AuNP-MCM seen in the TEM micrograph of Figure 1 vary greatly in diameter, but the particle specifically observed in Figure 1d under a smaller scale has a diameter of 20 nm.The SEM/EDS analysis in Figure 2a,b shows the gold nanoparticles on the mesoporous carbon with the gold concentration of 8.54% and carbon concentration of 10.73%.Figures 1b and 2a also show that the gold nanoparticles are evenly distributed on the MCM backbone.
The P-XRD spectrum seen in Figure 3 for the MCM showed a peak in the 20-30 • range, correlating to the carbon framework, and a peak at the 40-45 • range that also corresponds to the mesoporous carbon [25].After AuNPs were deposited on the MCM, the observed peaks of the MCM were slightly shifted, but they were still within the acceptable range.The peaks at 40-45 • disappeared in AuNP-MCM; the same phenomena were reported in some previous studies [26,27].The peaks at 38 • , 44 • , and 64 • corresponded with the (111), (200), and (220) lattice planes of gold nanoparticles [28].The P-XRD spectrum seen in Figure 3 for the MCM showed a peak in the 20°-to-30° range, correlating to the carbon framework, and a peak at the 40°-to-45° range that also corresponds to the mesoporous carbon [25].After AuNPs were deposited on the MCM, the observed peaks of the MCM were slightly shifted, but they were still within the acceptable range.The peaks at 40-45° disappeared in AuNP-MCM; the same phenomena were reported in some previous studies [26,27].The peaks at 38°, 44°, and 64° corresponded with the (111), (200), and (220) lattice planes of gold nanoparticles [28].

Catalytic Tests
Figure 4 shows the catalytic ability of AuNP-MCM at different reactant concentrations.The catalyzed reaction achieved the highest hydrogen generated rate of 0.0346 mL min −1 mg −1 at 1057 µmol, while the reaction achieved the lowest rate of 0.0140 mL min −1 mg −1 at 793 µmol.At the concentration of 952 µmol, the reaction rate was 0.0159 mL min −1 mg −1 .The hydrogen generated rate increased as the concentration of NaBH4 increased.The increase in the reactant concentration shifted the equilibrium of Equation ( 1) and led to the formation of more products [3,29].

Catalytic Tests
Figure 4 shows the catalytic ability of AuNP-MCM at different reactant concentrations.The catalyzed reaction achieved the highest hydrogen generated rate of 0.0346 mL min −1 mg −1 at 1057 µmol, while the reaction achieved the lowest rate of 0.0140 mL min −1 mg −1 at 793 µmol.At the concentration of 952 µmol, the reaction rate was 0.0159 mL min −1 mg −1 .The hydrogen generated rate increased as the concentration of NaBH 4 increased.The increase in the reactant concentration shifted the equilibrium of Equation ( 1) and led to the formation of more products [3,29].

Catalytic Tests
Figure 4 shows the catalytic ability of AuNP-MCM at different reactant concentrations.The catalyzed reaction achieved the highest hydrogen generated rate of 0.0346 mL min −1 mg −1 at 1057 µmol, while the reaction achieved the lowest rate of 0.0140 mL min −1 mg −1 at 793 µmol.At the concentration of 952 µmol, the reaction rate was 0.0159 mL min −1 mg −1 .The hydrogen generated rate increased as the concentration of NaBH4 increased.The increase in the reactant concentration shifted the equilibrium of Equation ( 1) and led to the formation of more products [3,29].The effect of pH on the catalyzed reactions was indicated in Figure 5.The reaction rate at pH 6 (0.0447 mL min −1 mg −1 ) was higher than that of pH 7 (0.0159 mL min −1 mg −1 ).The reaction was 0.0087 mL min mg −1 at pH 8.It had been stated in a previous kinetic study of sodium borohydride hydrolysis that the rate becomes lower as the pH increase due to the inhibition effect of OH − [29].The free proton at low pH condition accelerated the hydrolysis process [29].
Molecules 2024, 29, x FOR PEER REVIEW 6 of 13 The effect of pH on the catalyzed reactions was indicated in Figure 5.The reaction rate at pH 6 (0.0447 mL min −1 mg −1 ) was higher than that of pH 7 (0.0159 mL min −1 mg −1 ).The lowest reaction rate was 0.0087 mL min −1 mg −1 at pH 8.It had been stated in a previous kinetic study of sodium borohydride hydrolysis that the rate becomes lower as the pH increase due to the inhibition effect of OH − [29].The free proton at low pH condition accelerated the hydrolysis process [29].Scheme 1 shows a proposed mechanism for the overall reaction, which could explain these results.NaBH4 reduces when hydrolyzed and forms a reversable complex with the metal nanoparticle at a catalytic site.An adjacent catalytic site receives a hydride ion and stabilizes the nanoparticle-borohydride complex, which reacts with the water to release hydrogen gas.This process repeats until there are no longer any hydride ions available and the complex breaks, releasing tetrahydroxylborate and H2 gas.More acidic pHs increase the concentration of H + ions, which increase the conversion rate and supply of hy- Scheme 1 shows a proposed mechanism overall reaction, which could explain these results.NaBH4 reduces when hydrolyzed forms with the metal nanoparticle at a catalytic site.An adjacent catalytic site receives a hydride ion and stabilizes the nanoparticle-borohydride complex, which reacts with the water to release hydrogen gas.This process repeats until there are no longer any hydride ions available and the complex breaks, releasing tetrahydroxylborate and H 2 gas.More acidic pHs increase the concentration of H + ions, which increase the conversion rate and supply of hydroxide ions for the conversion.Figure 6 shows that the reaction rate of catalyzed hydrogen evolution reaction improved at higher temperature.The hydrogen generation rates were 0.0062 mL min −1 mg −1 , 0.0117 mL min −1 mg −1 , 0.0159 mL min −1 mg −1 , and 0.0232 mL min −1 mg −1 at 273 K, 288 K, 295 K, and 303 K, respectively.The same pattern was observed in other studies [3,4,29].From the Arrhenius plot seen in Figure 7 and Figure S2, the activation energy for the AuNP-MCM catalyst is determined to be 30 kJ mol −1 .The activation energy is also compared to previously recorded activation energies for other catalysts in Table 1, which shows it as one of the higher activation energies among other inorganic catalysts.AuNP- Figure 6 shows that the reaction rate of catalyzed hydrogen evolution reaction improved at higher temperature.The hydrogen generation rates were 0.0062 mL min −1 mg −1 , 0.0117 mL min −1 mg −1 , 0.0159 mL min −1 mg −1 , and 0.0232 mL min −1 mg −1 at 273 K, 288 K, 295 K, and 303 K, respectively.The same pattern was observed in other studies [3,4,29].Figure 6 shows that the reaction rate of catalyzed hydrogen evolution reaction improved at higher temperature.The hydrogen generation rates were 0.0062 mL min −1 mg −1 , 0.0117 mL min −1 mg −1 , 0.0159 mL min −1 mg −1 , and 0.0232 mL min −1 mg −1 at 273 K, 288 K, 295 K, and 303 K, respectively.The same pattern was observed in other studies [3,4,29].From the Arrhenius plot seen in Figure 7 and Figure S2, the activation energy for the AuNP-MCM catalyst is determined to be 30 kJ mol −1 .The activation energy is also compared to previously recorded activation energies for other catalysts in Table 1, which shows it as one of the higher activation energies among other inorganic catalysts.AuNP-MCM has higher activation energy than other catalysts, except the gold nanoparticle sup- From the Arrhenius plot seen in Figures 7 and S2, the activation energy for the AuNP-MCM catalyst is determined to be 30 kJ mol −1 .The activation energy is also compared to previously recorded activation energies for other catalysts in Table 1, which shows it as one of the higher activation energies among other inorganic catalysts.AuNP-MCM has higher activation energy than other catalysts, except the gold nanoparticle supported over multiwalled carbon nanotubes (Au/MWCNTs).When compared to unsupported gold nanoparticles (BCD-AuNPs), there was a marked improvement.This indicates that the addition of MCM improves the catalytic ability of gold nanoparticles.These results highly imply that AuNP-MCM is an efficient catalyst for sodium borohydride hydrolysis.
Molecules 2024, 29, x FOR PEER REVIEW 8 of 13 unsupported gold nanoparticles (BCD-AuNPs), there was a marked improvement.This indicates that the addition of MCM improves the catalytic ability of gold nanoparticles.These results highly that AuNP-MCM is catalyst sodium borohydride hydrolysis.

Catalytic Reusability Tests
The AuNP-MCM catalyst underwent reusability trials to determine its reusability after five consecutive uses under conditions involving 952 µmol of NaBH4, a pH of 7, and at

Catalytic Reusability Tests
The AuNP-MCM catalyst underwent reusability trials to determine its reusability after five consecutive uses under conditions involving 952 µmol of NaBH 4 , a pH of 7, and at 295 K.The plot in Figure 8 shows that the catalytic activity for the AuNP-MCM catalyst drastically decreased in catalytic activity after the second trial.From the third trial, the catalytic ability was increased and remained consistent in the fourth and fifth trials.This indicates that if more trials were to have been conducted, the catalytic activity may eventually remain consistent, resulting in a more ideal and durable catalyst.This would be consistent with Scheme 1, which shows the catalyst breaking away from the nanoparticle-borohydride complex to start the series of reactions again [47].However, these reusability trials indicate that while the nanoparticles greatly the catalytic activity of the developed catalysts, the AuNP-MCM catalyst is generally less stable.
Molecules 2024, 29, x FOR PEER REVIEW 9 of 13 295 K.The plot in Figure 8 shows that the catalytic activity for the AuNP-MCM catalyst drastically decreased in catalytic activity after the second trial.From the third trial, the catalytic ability was increased and remained consistent in the fourth and fifth trials.This indicates that if more trials were to have been conducted, the catalytic activity may eventually remain consistent, resulting in a more ideal and durable catalyst.This would be consistent with Scheme 1, which shows the catalyst breaking away from the nanoparticleborohydride complex to start the series of reactions again [48].However, these reusability trials indicate that while the nanoparticles greatly increase the catalytic activity of the developed catalysts, the AuNP-MCM catalyst is generally less stable.

Synthesis
The Mesoporous carbon was reported to be synthesized from starch via the Starbon synthesis method [21,22,24].Nitrogen sorption/desorption isotherms generated at 77 K for mesoporous carbon is presented in Figure S1 and nitrogen adsorption data for mesoporous carbon is included in Table S1.Gold nanoparticles were synthesized by bringing a 1 mM aqueous chlorauric acid (Sigma Aldrich, St. Louis, MO, USA) solution to a boil and adding 1% aqueous sodium citrate (Sigma Aldrich) dropwise for five minutes with continuous stirring with a magnetic stir bar [49].The AuNP-MCM nanocomposite was synthesized by adding 40 mL of the nanoparticle solution to 1 g of mesoporous carbon in order to functionalize the mesoporous carbon via the incipient wetness impregnation method [4,5].The resulting precipitate was filtered and dried in a vacuum oven at 100 °C for 24 h.

Characterization
Transmission electron microscopy (TEM, JEM-2100F, JEOL, Akishima, Tokyo, Japan) was used to visualize the size of the nanoparticles in the composite and to characterize the binding between the nanoparticles and the mesoporous carbon.These samples were prepared by putting 1 µL of the nanoparticle solution onto the TEM sample grid and letting it dry overnight.X-Ray diffraction (XRD, Rigaku Miniflex II, Cu Kα X-ray, nickel filters, Rigaku, Tokyo, Japan) was used to determine the crystal structure of mesoporous carbon Hydrogen Generated (mL)

Synthesis
The Mesoporous carbon was reported to be synthesized from starch via the Starbon synthesis method [21,22,24].Nitrogen sorption/desorption isotherms generated at 77 K for mesoporous carbon is presented in Figure S1 and nitrogen adsorption data for mesoporous carbon is included in Table S1.Gold nanoparticles were synthesized by bringing a 1 mM aqueous chlorauric acid (Sigma Aldrich, St. Louis, MO, USA) solution to a boil and adding 1% aqueous sodium citrate (Sigma Aldrich) dropwise for five minutes with continuous stirring with a magnetic stir bar [48].The AuNP-MCM nanocomposite was synthesized by adding 40 mL of the nanoparticle solution to 1 g of mesoporous carbon in order to functionalize the mesoporous carbon via the incipient wetness impregnation method [4,5].The resulting precipitate was filtered and dried in a vacuum oven at 100 • C for 24 h.

Characterization
Transmission electron microscopy (TEM, JEM-2100F, JEOL, Akishima, Tokyo, Japan) was used to visualize the size of the nanoparticles in the composite and to characterize the binding between the nanoparticles and the mesoporous carbon.These samples were prepared by putting 1 µL of the nanoparticle solution onto the TEM sample grid and letting it dry overnight.X-ray diffraction (XRD, Rigaku Miniflex II, Cu Kα X-ray, nickel filters, Rigaku, Tokyo, Japan) was used to determine the crystal structure of mesoporous carbon and its composite.The sample was put onto a P-XRD slide, and a Rigaku Miniflex II

Figure 2 .
Figure 2. SEM/EDX analysis with (a) the SEM micrograph of the AuNP-MCM catalyst at scales of 2 µm and (b) the EDS spectrum correlating to the indicated gold nanoparticle.

Figure 2 .
Figure 2. SEM/EDX analysis with (a) the SEM micrograph of the AuNP-MCM catalyst at scales of 2 µm and (b) the EDS spectrum correlating to the indicated gold nanoparticle.Molecules 2024, 29, x FOR PEER REVIEW 5 of 13

Figure 3 .
Figure 3. P-XRD spectra for the MCM and AuNP-MCM.The asterisks highlight the location of important peaks.

Figure 3 .
Figure 3. P-XRD spectra for the MCM and AuNP-MCM.The asterisks highlight the location of important peaks.

Figure 5 .
Figure 5. Volume of hydrogen generated over time in the hydrogen evolution reactions catalyzed by AuNP-MCM at different pHs (pH 6, pH 7, and pH 8).

Figure 5 .
Figure 5. Volume of hydrogen generated over time in the hydrogen evolution reactions catalyzed by AuNP-MCM at different pHs (pH 6, pH 7, and pH 8).

Figure 6 .
Figure 6.Volume of hydrogen generated over time in the hydrogen evolution reactions catalyzed by AuNP-MCM at different temperatures (273 K, 288 K, 295 K, and 303 K).

Scheme 1 .
Scheme 1. Proposed mechanism for the hydrolysis of NaBH 4 while catalyzed by AuNP-MCMs.

Figure 6 .
Figure 6.Volume of hydrogen generated over time in the hydrogen evolution reactions catalyzed by AuNP-MCM at different temperatures (273 K, 288 K, 295 K, and 303 K).

Figure 6 .
Figure 6.Volume of hydrogen generated over time in the hydrogen evolution reactions catalyzed by AuNP-MCM at different temperatures (273 K, 288 K, 295 K, and 303 K).

Figure 7 .
Figure 7. Arrhenius plot for calculating the activation energy of a hydrogen generation reaction with the AuNP-MCM catalyst.

Figure 7 .
Figure 7. Arrhenius plot for calculating the activation energy of a hydrogen generation reaction with the AuNP-MCM catalyst.

Figure 8 .
Figure 8. Testing reusability of the AuNP-MCM catalyst after five consecutive hydrogen generation reactions.

Figure 8 .
Figure 8. Testing reusability of the AuNP-MCM catalyst after five consecutive hydrogen generation reactions.

Table 1 .
Comparison of reported activation energies for catalyzed NaBH4 hydrolysis.

Table 1 .
Comparison of reported activation energies for catalyzed NaBH 4 hydrolysis.